In fuel cell-supported transportation systems so-called chemical reformers are used for obtaining the required hydrogen from hydrocarbon-containing fuels.
The optimal operating temperature of a chemical reformer is usually far above its surrounding temperature. Particularly in vehicles for personal transportation, this leads to problems. The numerous standstill phases of the vehicle lead to a large number of cold start phases, in which especially the chemical reformer does not function optimally. At very slight load, the reformer also does not, under certain circumstances, achieve the optimal operating temperature by the heat arising in it, or loses it during the operation.
Therefore, particularly in the case of fuel cell supported drive systems having a chemical reformer, it may be of advantage to install afterburner devices which rapidly bring the chemical reformer to operating temperature using the heat produced by it, and/or of using accumulated residual gases thermally.
An afterburner device burns the combustible residual gases, such as residual hydrogen, while forming flames and/or at least partially catalytically, and is thermally coupled to the chemical reformer. However, the heat energy of the combustible residual gases is generally not, by itself, sufficient for making available a sufficiently great heat output. That is why generally, in addition or by itself, fuel is metered into the afterburner device. In this context, the fuel, which is preferably present in liquid form, is injected into a combustion chamber, finely divided, by devices that are costly and subject to error, as a cloud of droplets having a droplet diameter that is as small as possible. The slight droplet diameter is required in order to bring the fuel into contact with oxygen and heat over as large a surface as possible, and in order thus to carry out the combustion process as completely as possible.
In this context, it may be a disadvantage that metering devices for generating a cloud of droplets having a small droplet diameter are very costly, cost intensive and subject to error. The required small droplet diameter is often able to be achieved only by using high fuel pressure, the generation of high pressure requiring a relatively high quantity of power, and in particular, the equipment for generating the pressure requiring much space. In addition, such metering devices usually have very small metering apertures, which, by combustion residues or deposits, change the metering behavior of the metering device in an impermissible and poorly controllable way. Alternatively to, or supportive of the application of high fuel pressure, solutions having air support are known for the fine atomization of the fuel, the fuel or the residual gas being intermingled sufficiently long with air before the combustion. In this connection, the disadvantages are the large space requirement, the control of the air metering that is costly and subject to interference, and the additional energy requirement.
Finally, especially at low power, the danger arises of unexpected extinguishing of the open continually burning flame in the combustion chamber. The heat output of the afterburner device is therefore strongly restricted in the downward direction. Furthermore, there is always required a certain amount of time for shutting off the fuel supply or reigniting the flame. During this time, the fuel and the residual gas are able to collect in the combustion chamber. This negatively influences the reignition, a catalytic converter that may possibly be present may be damaged, and uncombusted fuel and residual gas may escape into the atmosphere. In spite of all the measures named, uncombusted or incompletely combusted portions remain behind in the exhaust gas of the afterburner device, which are, in part, poisonous or chemically aggressive. This leads to increased environmental loading and material loading, and besides all that, the caloric value of the fuel or the residual gas is utilized only incompletely.